Don't hide a slider if it's already being shown in the VolumeDialog

As the VolumeDialogImpl we want to continue showing a slider if it has
already being shown in the dialog.

Flag: com.android.systemui.volume_redesign
Bug: 369992924
Test: atest VolumeDialogSlidersInteractorTest
Change-Id: I8a6c10f60fb2ac9bcb9d1e7688d4d233ef7d775f